Heat transfer ink ribbons are conventionally put into wide use in order to print characters or bar-code images on transfer mediums such as cut papers, labels and cards. They have commonly a structure comprising, as shown in FIG. 1A, a base material 1 made of polyester or the like and formed on one side thereof a hot-melt ink layer 2 comprised of a colorant and a binder such as wax.
Now, when stain resistance and solvent resistance are required on transferred images such as characters and bar-code images, thermoplastic resins such as polyester resins (Japanese Patent Application Laid-open No. 5-16535, etc.) or vinyl chloride resins (Japanese Patent Application Laid-open No. 7-76178, Japanese Patent Publication No. 3-18837, etc.) are used as the binder of the hot-melt ink layer 2 of the heat transfer ink ribbon in place of, or together with, the wax. Heat transfer ink ribbons having such an ink layer are called resin type ribbons.
Such resin type ribbons are characteristic of an ink layer 2 having a high toughness. Hence, transferred images formed from this ink layer 2 can be expected to have a high stain resistance and solvent resistance.
However, the transfer performance to transfer mediums tends to lower depending on the proportion of thermoplastic resin in the ink layer 2. This tendency is remarkable especially when not paper but plastic labels or cards having good durability and solvent resistance are used as transfer mediums.
Accordingly, when resin type ribbons are produced, it is common to form the ink layer 2 in a thickness as small as 1.0 .mu.m or below and also to form between the base material 1 and the ink layer 2 an intermediate layer 3 (FIG. 1B) capable of undergoing cohesive failure at the time of heat transfer or a release layer (not shown) capable of being peeled, to thereby improve transfer sensitivity.
However, conventional resin type ribbons have problems on heat transfer performance, solvent resistance or printer adaptability. More specifically, as stated above the ink layer 2 of resin type ribbons is formed in so small a thickness that the ink layer 2 must be incorporated with the colorant in a large quantity so as not to cause a low image density. Hence, there is a problem that the resin type ribbons have a low heat transfer performance or solvent resistance. Thus, it is very difficult to use the resin type ribbons in the field where images are required to be accurate as in bar-code images.
In instances where plastic labels or cards are used as transfer mediums, resins having a good adhesion to them, which usually are thermoplastic resins of the same type as the thermoplastic resins used in the labels or the like, must be selected as binders of the ink layer 2. Since, however, labels have numberless kinds, it is troublesome to change resin type ribbons when labels are changed.
With regard to labels comprised of polyolefin resins such as polyethylene or polypropylene, having a low chemical polarity, or matte film labels with a great surface roughness, it is difficult to improve the heat transfer performance of the ink layer 2 without regard to what types of binders are used in the ink layer 2.
Meanwhile, recently, as shown in FIG. 2, what is called an edge-face head type printer, a heat transfer printer whose heating element 21 is formed at a side edge of a thermal head substrate 22 in the direction of the movement of an ink ribbon 23, attracts notice as a printer that can improve the heat transfer performance of heat transfer ink ribbons. This printer attempts to improve the transfer performance of the ink layer by providing a greater angle .theta. at which the ink ribbon 23 is peeled from the transfer medium 24, than that of conventional printers, in a hot-molten state or heat-softened state where the ink layer stands in a low cohesive force.
When, however, conventional resin type ribbons are applied in this edge-face head type printer, there has been the problem that the ink layer 2 is not sharply separated and transferred from the base material 1, but the ink layer 2 causes cohesive failure in it, resulting in a low transfer density of the images obtained.
The present invention solves the above problems the prior art has had. Accordingly, an object of the present invention is to provide a heat transfer ink ribbon that can exhibit a good heat transfer performance also on labels with a low chemical polarity, e.g., polyolefin type ones, and on matte labels with a rough surface, and yet can form high-quality images with a good stain resistance and solvent resistance even when the edge-face head type printer is used.